In the last few decades, Salmonella enterica serovar Enteritidis has emerged as a major cause of food-borne illness worldwide. This pathogen is distinguished from its many close relatives also found in poultry environments by its ability to infect chicken ovaries before the eggshell is formed, allowing transmission through intact eggs. Once established in the human host from raw or undercooked eggs or egg products, this bacterium causes gastroenteritis similar to other Salmonella enterica serovars. Infection in poultry flocks, which is asymptomatic, was first noticed in the late 1970's, and in the 1980's spread rapidly throughout the United Kingdom, the United States, South America, and other areas. During this period, the proportion of salmonellosis cases attributed to Salmonella serovar Enteritidis increased substantially, showing a 275-fold increase in Argentina and becoming the predominant cause of this disease in the U.S. (see Hogue, A et al. 1997, Epidemiology and control of Salmonella enteritidis in the United States of America, Revue Scientifique et Technique 16:542-553, Morales, R. A. et al 1999, Economic Consequences of Salmonella enterica Serovar Enteritidis Infection in Humans and the U.S. Egg Industry, Iowa State University Press, Ames, Rodrigue, D. C. et al. 1990, International increase in Salmonella enteritidis: a new pandemic? Epidemiol. Infect. 105:21-27). Baumler et al. suggested that this rapid increase of Salmonella serovar Enteritidis may have been due to successful campaigns to eradicate Salmonella serovars Pullorum and Gallinarum, the causative agents in chickens of bacillary white diarrhea and fowl typhoid, respectively ( see Bäumler, A. J., et al. 2000, Tracing the origins of Salmonella outbreaks, Science, 287:50-2). It is hypothesized that these avian-adapted Salmonellae provided cross-immunity against Salmonella serovar Enteritidis because of important similarities in lipopolysaccharide structures. Therefore, these campaigns may have opened an ecological niche that has since been occupied by Salmonella serovar Enteritidis. This view remains controversial, however, as serovars Gallinarum and Pullorum remain prevalent in many developing countries where serovar Enteritidis has nevertheless increased dramatically, and turkey flocks in developed countries, now free of serovars Gallinarum and Pullorum, have not been colonized by serovar Enteritidis (see Pomeroy, B. S. et al. 1991, Fowl typhoid, In: Calnek, B. W., Barnes, H. J., Beard, C. W. et al. [eds.]; Diseases of Poultry. Ames, IA: Iowa State University Press, pp. 100-7, Silva, E. N. 1985, Salmonella gallinarum problem in Central and South America. In: Snoyenbos, G. H. [ed], and Proceedings of International Symposium on Salmonella, New Orleans, La. American Association of Avian Pathologists, Kennett Square, Pa., pp. 150-6). Unlike the avian-adapted Salmonellae, rodents serve as an animal reservoir for Salmonella serovar Enteritidis, suggesting that culling would not be an effective method of control. It is possible that the use of Salmonella serovar Enteritidis as a rodenticide may have contributed to the current prevalence of this serovar, and it is also likely that infected rodents are currently a source of disease. In addition to the health risks, this pathogen has had a significant economic impact on the egg industry through decreased consumer confidence following well-publicized outbreaks.